For convenience, the following description will be mainly directed to mirrors and improving the corrosion resistance of the reflective layer used to make the mirror but it will be understood by those skilled in the art that other articles (e.g., metal containing substrates and metal particles) may be treated using the method of the invention to enhance the properties of the articles, e.g., the corrosion resistance of the metal.
Conventional mirrors can be made by a process in which a thin layer of a reflective metallic film is applied onto a substrate, typically glass or plastic. Mirrors are generally made continuously by a sequence of steps on a mirror conveyor. The first step lightly polishes and cleans the glass surface and after rinsing, the next step sensitizes the surface e.g. with an aqueous stannous chloride solution. The metal deposited on the substrate is typically silver, although other metals or metal compositions may be used. The silver film layer is deposited on the sensitized glass surface by one of many methods such as described in U.S. Pat. No. 4,737,188 to Bahls. In one typical process, an ammoniacal silver nitrate solution and a reducing agent solution containing a strong base are sprayed on and combined at the sensitized glass surface to deposit the silver film.
If air, water, contaminants, chemicals such as ammonia and other glass cleaners, or other compounds, come in contact with the reflective metallic film, the film may corrode, oxidize, or lose contact with the glass. To protect the film and reduce damage to the metallic layer, a protective overlay of copper may be applied over the metallic film, and/or a protective paint may be applied. This copper film may be applied to and over the silver film by any of a variety of prior art procedures such as a galvanic process which utilizes an aqueous suspension of iron powder and an aqueous solution of copper sulfate or by the disproportionation of cuprous ions on the silver surface. The latter process is described in U.S. Pat. No. 5,419,926 to Soltys. The copper layer is normally painted to produce the finished mirror, or another protective coating such as a hardened organic resin incorporating a corrosion inhibitor may likewise be applied as shown in U.S. Pat. No. 5,156,917 to Sanford. A standard mirror making process thus comprises a series of steps which steps are performed in sequence on a conveyor as part of a continuous mirror making process.
Historically, a serious problem of the mirror manufacturing industry was the need for the copper layer on the silver layer to inhibit corrosion of the silver and increase adhesion of the paint. The application of copper to the silver surface necessarily produces copper containing waste streams which must be environmentally treated or processed for recycling. Typically, the copper streams are treated to remove copper before discharge to the effluent and this procedure is complex and costly. The copper film on the mirror is also a weak link in the life of a conventional mirror. The copper film is easily corroded when the mirror is subjected to ammonia or alkaline glass cleaners because these cause the edges of mirrors to corrode and turn black in color thereby shortening the life of the mirror.
In the seventies researchers found a method of improving the adhesion of metallic silver or copper films on surfaces like glass (e.g. A. Ya. Kuznetsov, et al. in Sov. J. Opt. Technol. 42, 1975, 604). By “activating” a glass surface sensitized with tin dichloride with a PdCl2 solution a markedly improved adhesion has been recognized. Furthermore, it has been realized that corrosion protection may also be enhanced by improving adhesion between the layer of the reflective metal and the substrate. This finding helped to overcome the above mentioned problem of the mirror manufacturing industry. Due to the improved adhesion of the metallic silver film on the substrate the need for protective coatings (e.g. the copper layer mentioned above) overlying the metallic layer has been eliminated. The mirror manufacturing industry now used Pd-based activation (supersensitizing) to produce so called “copper-free mirrors”.
A number of patents have been issued for improving treatment of the silver surface in the mirror making process to further increase corrosion resistance. In U.S. Pat. No. 5,374,451 to Servais et al., a mirror is shown having a reflective layer of silver which has been treated with a solution containing ions of at least one of the group consisting of: Cr (II); V (II or III); Ti (III or II); Fe (II); In (I or II); Cu (I); and Al (III). The solution may alternatively contain Sn (II) ions.
In U.S. Pat. No. 5,235,776 to Servais et al., a reflective article is described including a glass substrate and a reflective metal coating, preferably silver, deposited on the glass substrate. This reflective layer of silver has been treated with an acidified aqueous solution of a stannous salt. This solution has to be free from opalescence and, therefore, needs to be freshly-made. By this treatment a population of tin atoms is augmented in a surface stratum of the silver layer. As a preferred embodiment a silane treatment has been applied to the metal coating before a final outside coating e.g. a paint layer is applied.
Alternatively, it has been proposed to protect the silver coating by treatment with an acidified aqueous solution of Sn (II) salt (see British patent application GB 2252568). According to another proposal, the silver coating is protected by treatment with a solution containing at least one of Cr (II), V (II or III), Ti (II or III), Fe (II), In (I or II), Cu (I) and Al (III) (see British patent application GB 2254339).
Furthermore, it is known to apply a tin precipitate on top of the silver layer, which optionally can be further treated with a silane solution, see, e.g., U.S. Pat. Nos. 6,017,580; 6,218,019; and 6,432,200 (Soltys). Alternatively the silver layer can be protected by a treatment with a tin salt and a silane.
Unfortunately, current methods of treating surfaces of reflective metal coatings to inhibit corrosion thereof are quite costly and in some cases involve the application of a two component system. Furthermore, current methods make use of heavy metals like tin which due to environmental concerns should be restricted as much as possible.
Bearing in mind the problems and deficiencies of the prior art, in one embodiment it is an object of the present invention to provide a method for enhancing the corrosion resistance of a reflective metal coating deposited on a substrate wherein the above mentioned prior art processes are replaced with a less expensive and an environmentally friendly one component system which not only protects the deposited metal layer against corrosion but also enhances the adhesion of an optional final outside coating e.g. a paint commonly used in the art. The inventive process e.g. may be used in existing commercial mirror making conveyor systems without having to add extra sections to the system.
In another embodiment it is an object of the invention to provide an improved corrosion inhibiting metal coated substrate e.g. a mirror and other metal substrates and metal articles of manufacture.
Still other objects and advantages of the invention will be apparent from the specification.